This invention relates to a new and improved circuit breaker, and more particularly to a new and improved circuit breaker of the thermostatic type with a magnetic assist for applications in which it is desired to interrupt an electrical circuit under predetermined conditions. Circuit breakers of the type described in U.S. Pat. No. 3,697,915 have found wide acceptance in the marketplace and are very effective for many applications, however, they are limited in their ability to close on a very high current fault without failing by having the contacts weld shut or by having excessive arc erosion.
It is an object of the invention to provide a new and improved circuit breaker particularly suitable for closing on high current faults by eliminating contact bounce or electrodynamic separation, to provide additional contact force without adding to unlatch forces and to provide a circuit breaker where the amount of assist or additional contact force is automatically regulated by the demand or level of fault current. Another object of this invention is to provide a circuit breaker in which interrupting capacity is not significantly sensitive to closure velocity and to provide an assist system that does not effect trip time and therefore maintains inherent coordination capability. Yet another object of the invention is to provide a circuit breaker which is economical to manufacture and is readily adapted to mass production and to provide such a circuit breaker which is of small size, light weight and exceedingly compact structure. Other objects will be in part pointed out hereafter.
Known prior art miniature circuit breakers are limited in their ability to close on high current faults. In such breakers as the movable contacts close on a high current fault during a reset operation, the movable contacts pass through a point of zero contact force at which point the high current fault causes the contacts to bounce apart with severe arcing resulting. Such arcing results in melting of metal on the contact surface. As the reset operation is finished and the contact force is biased toward closure, the contacts are again brought together, but this time they may weld together when the melted material on the contact surface solidifies. Of course, this welding action could prevent the trip mechanism from separating the contacts and a failure could occur.
The present invention uses a magnetic assist employing an iron core pole piece and an armature to prevent contact bouncing. An iron core is positioned between two stationary contacts of a conventional circuit breaker and is insulated with dielectric material so that the current path of the circuit breaker forms a partial loop about the core. An armature of a magnetic material is affixed to the back plate of the movable contacts. Then, when closing on a current fault, the current flowing around the core will cause a magnetic field which in turn will exert an instant force on the armature in the closing direction of the movable contacts and will prevent them from bouncing or magnetostrictively being forced back. The amount of force exerted by the magnetic field is proportional to the amount of current flowing through the circuit breaker and therefore can be easily calibrated to work in coordination with the thermostatic type circuit breaker so as to only be effective when large amounts of current pass through the circuit breaker.
The invention accordingly comprises the elements and combinations of elements, features of construction and arrangements of parts which will be exemplified in the structures hereinafter described and the scope of the application of which will be indicated in the appended claims.